Summary translation

Soils of farms managed organically for more than 30 years contained significantly less P and K than soils of the farms managed organically for less than 5 years. However, potato yields were not different and ranged between 20 and 50 t/ha fresh matter, mean 30 t/ha. The yields were related to texture soil, fertilization and livestock units per hectare (R2=0.55). The resistance against P. ultimum correlated positively both with the amount of manure and with the ratio basal respiration to microbial biomass (R2=0.61). Resistance against P. infestans correlated with the proportion of cereals in the crop rotation and the C/N-ratio of the microbial biomass in the soil (R2=0.74). Conclusions These data show that fertilization, crop rotation and community structure influence pathogen suppression.